I am running twin 292's in my boat right now. You don't need to build anything above stock for your engines to run reliably for 2500 hours. My boat is a little bigger than the 25 Bertram and a little heavier, but not much. I run my boat on regular gas and cruise it at 3200 RPM's. The biggest killer of a marine engine is ROT. It will likely take 10 years or more for you to get 2500 hours on your boat, and that is if your use it every weekend of the year for 10 years. Sitting in the marine environment is the killer.
You WILL burn more fuel than you are hoping for. I have 2-barrel carbs on my engines and burn ~8 GPH per engine at 3200 RPM's. My speed is ~30 mph.
Any other info I can provide, let me know.
